1. Field of the Invention
The invention relates to vehicle mounted satellite antennae. More particularly, the invention relates to a vehicle mounted satellite antenna which is easy to install, has a low profile, and which is operable while the vehicle is in motion.
2. State of the Art
It has long been known to mount a satellite antenna (dish) atop a vehicle for purposes of communicating with a geostationary or other type of satellite. The initial applications for mounting a satellite dish on a vehicle were military communication and remote television news broadcasting. Consequently, the first methods of mounting a satellite dish included a telescoping mast which was hingedly coupled to the vehicle. When the vehicle was in motion, the mast would be retracted and folded with the satellite dish lying end up on the roof or a side wall of the vehicle. The dish would be deployed only when the vehicle was stationary. Such a deployable vehicle mounted satellite dish is disclosed in U.S. Pat. No. 5,961,092 to Coffield. Until recently, no vehicle mounted satellite antennae were operable while the vehicle was in motion. The relatively large size of a conventional satellite dish antenna presents significant wind resistance if deployed on a vehicle in motion. This wind resistance adversely affects the operation of the vehicle and subjects the satellite dish to potential wind damage. Moreover, satellite dishes must be accurately aimed at a satellite within a relatively narrow aperture or xe2x80x9clook windowxe2x80x9d. In order to operate a satellite dish mounted on a vehicle in motion, it would be necessary to constantly re-aim the dish in order to maintain communication with the satellite.
Recently, satellite antennae have been developed which may be deployed on a vehicle and operated while the vehicle is in motion. Such antennae are disclosed in U.S. Pat. No. 5,398,035 to Densmore et al., U.S. Pat. No. 5,982,333 to Stillinger, and U.S. Pat. No. 6,049,306 to Amarillas. These antenna systems generally include a satellite antenna of reduced size and a solenoid system for aiming the antenna. The solenoid system is coupled to a feedback system and/or vehicle motion detectors in order to automatically re-aim the antenna as the vehicle is in motion. In order to reduce aerodynamic drag and protect the antenna from wind damage, an aerodynamic radome is often used to cover the antenna.
Vehicle mounted satellite antennae which are operable while the vehicle is in motion, can provide one-way or two-way satellite communications. Some applications for such antennae include satellite television reception, telephony in remote locations where cellular telephone service is unavailable, and broadband data communications. The application of television reception may be advantageously applied in common carrier transportation such as long distance buses, in recreational vehicles including boats, and in the rear seats of family mini-vans. The application of remote telephony may be applied in the same situations as well as in various other governmental and commercial settings. The application of broadband data communication may also be applied in many personal, commercial, and governmental settings.
Broadband satellite communication, such as television reception or broadband data communication, requires a high gain antenna with high cross-polarization isolation and low signal sidelobes. Satellite antenna gain is proportional to the aperture area of the reflector. Stationary satellite antennae typically utilize a circular parabolic reflector. Satellite antennae designed for use on a moving vehicle have a low profile. In order to maintain gain, these low profile antenna are short but wide so that the overall aperture area is kept high. However, this design strategy only works to a point. When the width to height ratio exceeds a certain value such as 2, the efficiency of the antenna is adversely affected. The presently available vehicle mountable satellite antenna for commercial and personal use are no shorter than approximately fifteen inches in height.
In addition to the issue of providing low profile tracking antennae, the process of installing a satellite antenna on a vehicle is not trivial. Holes must be drilled through the roof (or body panel) of the vehicle; coaxial cable must be routed from the antenna to a receiver or transceiver; and power cables must be routed to the antenna""s tracking system. The installation process is therefore time consuming and costly.
It is therefore an object of the invention to provide a vehicle mountable satellite antenna.
It is also an object of the invention to provide a vehicle mounted satellite antenna which is operable while the vehicle is in motion.
It is another object of the invention to provide a vehicle mounted satellite antenna which has a low profile.
It is also an object of the invention to provide a vehicle mounted satellite antenna which has high gain.
It is another object of the invention to provide a vehicle mounted satellite antenna which has high efficiency.
It is still another object of the invention to provide a vehicle mountable satellite antenna which is easy to install.
In accord with these objects which will be discussed in detail below, the satellite antenna of the present invention includes two low profile paraboloid linear reflector antenna assemblies mounted on a rotatable platform which is rotatably coupled to a base plate. Each antenna assembly is provided with two sub-reflectors with a plastic matching element between them. The two antenna assemblies are mounted parallel to each other and are pivotable relative to the rotatable platform. A first servo motor is coupled to the rotatable platform for azimuth tracking. A second servo motor is coupled by a rigid arm to both antenna assemblies for elevation tracking. The two antennae assemblies are each provided with a line feed for receiving a polarized satellite signal. A number of slot antenna probes are located in the back of each antenna assembly. The signal is coupled from the slot antenna into a microwave PCB or waveguide in the back of each antenna. The antenna probes are attached to a microwave circuit board, where two orthogonal linearly polarized signals are extracted. The two linearly polarized signals are fed into a 90xc2x0 hybrid and two circularly polarized signals are extracted. The signals of the same circular polarization from the same antenna assembly are amplified and combined into a single signal in a beam forming network (BFN) circuit on the microwave PCB.
In order to correct for time delay difference in the signals received by the two antenna assemblies, a phase shifter is employed to correct for the phase shift for the signal received from one antenna before it is combined with the other antenna. A unique feature of this antenna design is that only one phase shifter is required, thereby achieving a very low cost design as compared to the conventional phased array antenna implementation which typically requires a large number of phase shifters.
According to an alternate embodiment, the backside of each antenna dish is provided with a rectangular wave guide structure with a step tooth polarizer stud in the middle of the wave guide. The polarizer stud within the rectangular wave guide converts the signal from linear polarization to circular polarization. Each antenna contains two rows of multiple antenna feeds distributed over the entire length of the antenna. The upper row of antenna feeds extracts a (left or right) circularly polarized signal and the lower row of antenna feeds extracts a (right or left) circularly polarized signal. Each row of antenna feeds is connected via a circuit board or wave guide to a beam forming network (BFN) where signals are amplified and combined into a single signal. The output of one of the BFNs is connected to the input of a phase shifter via a flexible coaxial cable. The output of the other BFN is connected to either an attenuator or an amplifier (depending on whether the phase shifter amplifies or attenuates the other signal) and then to one input port of a two-to-one combiner via a flexible coaxial cable. The output of the phase shifter is connected to the other input port of the combiner. The amplifier or attenuator is used to amplify or attenuate the signal by the same amount as the gain or loss of the phase shifter so that the power of the signals from both BFN""s are equal before they are combined.
Dividing the antenna physical aperture into two or more paraboloid linear dishes reduces the overall height of the antenna array by half. Providing each cylindrical dish with multiple feeds instead of single feed maintains the overall antenna efficiency.
The combined signal from the two paraboloid linear antennae is routed through a rotary joint, which routes the received signal to circuits located under the rotatable platform but above the base plate. According to the preferred embodiment of the invention, the circuits between the rotatable platform and the base plate include a re-transmitter for transmitting received satellite signals (at a longer wavelength) to a first receiver inside the vehicle. A second receiver is also preferably provided on the base plate. According to one embodiment of the invention, the second receiver is used to receive channel selection signals and other control signals transmitted by a transmitter inside the vehicle. According to another embodiment, a transceiver is used at the base plate to provide two-way wireless communication with equipment, such as telephones and computers, through another transceiver inside the vehicle.
The use of the re-transmitter and second receiver between the rotatable platform and the base plate eliminates the need for signal wiring between the antennae assembly and the interior of the vehicle. According to a preferred embodiment of the invention, an independent power supply is also provided between the rotatable platform and the base plate to eliminate the need for power wiring between the antennae assembly and the interior of the vehicle. According to one preferred embodiment, the independent power supply includes a storage device such as a battery or a coil and a charging device such as a wind powered generator. A solar cell array may also be used as a charging device.
According to other aspects of the invention, electronic dithering systems are used to track a satellite quickly while a vehicle is in motion. Methods are also provided for adjusting the bias of motion sensors via the use of longitudinal and lateral accelerometers. Methods are also provided for receiving either circularly polarized or linearly polarized signals. According to one embodiment of the invention, the xe2x80x9cdata portxe2x80x9d of a conventional satellite receiver settop box is used determine the appropriate phase shift in the antennae array for a selected channel.
According to another aspect of the invention, the antenna system is provided with a retractable radome. When the antenna is not in use, the two cylindrical dishes are aimed straight up, decreasing the overall height of the system, and the radome is retracted.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.